The 1990 Clean Air Act Amendments mandated that gasoline suppliers reformulate their gasoline blends to be cleaner burning, and to have less impact to the atmosphere. In response to this mandate, gasoline suppliers began to blend their fuels with oxygenate chemicals, such as alkyl ethers. In particular, methyl-tert-butyl ether (hereinafter “MTBE”), was used quite extensively, and often comprised up to 10 to 15% by volume of unleaded gasoline.
Now, having used oxygenated fuels for several years, it has become clear that these cleaner-burning fuels pose great threats to groundwater resources. In particular, many oxygenate chemicals are very soluble in water and are slow to degrade in the environment; hence they tend to accumulate in water resources once released to the environment. For example, MTBE has been detected in groundwater with high frequency in many states and there are well-documented cases of impacts to municipal water supply wells. In some cases these impacts result from accidental gasoline spills; in other cases they are attributed to the re-deposition of chemicals emitted to the atmosphere from partially combusted automobile exhaust.
It is also now known that when oxygenate chemicals including alkyl ethers, such as MTBE and tertiary butyl alcohol (hereinafter “TBA”), are found in the subsurface, then they are resistant to biodegradation under natural conditions. This is the main reason for their persistence and accumulation in soil and groundwater.
With the increase in our knowledge of the behavior of these chemicals, and the increase in documented impacts, regulatory agencies are now vigorously enforcing cleanup standards for MTBE and TBA in groundwater. As a result, practitioners are searching for technologies that can clean-up soil and aquifers contaminated with MTBE and TBA.
Shallow contaminated soil can be treated by excavating the contaminated soil and then treating it above-ground. However, in most cases, it is preferable to treat contaminated soils in situ so as to minimize disturbance of the site and prevent further release of the contaminants to the atmosphere. Along these lines, the invention described by Visser et al. (U.S. Pat. Nos. 4,593,760 and 4,660,639) has been used at some MTBE-impacted sites with success. Visser's process relates to the extraction of soil gas vapors from the subsurface. However, this process is limited in applicability to permeable soils located above the water table, and is limited to volatile alkyl ethers. Furthermore, by itself, it is a non-destructive process that must be coupled with above-ground treatment (such as thermal oxidation) if the alkyl ethers or TBA are to be destroyed.
While there is little performance data available, it is thought by practitioners that contaminated groundwater might be remediated by application of either pump-and-treat technology or in situ air sparging technology. In the former, contaminated groundwater is withdrawn by pumping from groundwater wells and is purified above-ground. Following purification, the groundwater is reinjected to the aquifer or discharged above-ground. A groundwater pump-and-treat system is viewed as both ineffective and expensive because it is maintenance-intensive, operations often are on the scale of decades, and it merely transfers contamination from the aqueous phase to the atmosphere, to a solid medium for later disposal, or to a surface treatment facility. In situ air sparging technology is described by Billings et al. (U.S. Pat. No. 5,221,159). In that invention, air is injected in situ into the contaminated groundwater with the hope that groundwater contaminants will be volatilized or that the addition of oxygen will help the aerobic biodegradation of readily biodegradable contaminants. With respect to application to oxygenate chemicals such as alkylethers and TBA, indigenous organisms capable of biodegrading these chemicals are not always present; if they are present, it is usually at such low numbers that the in situ air sparging process can practically only cause the volatilization of the alkylethers and TBA. Thus, application of this process would only cause the transfer of alkylethers and TBA from the groundwater to soil gas and the atmosphere. In some cases the vapors are collected and treated above-ground, but again, above-ground treatment of vapors is typically very expensive and problematic.
U.S. Pat. No. 5,874,001, issued Feb. 23, 1999, proposes a method for removing contaminants from ground water or soil by injecting oxygen gas into ground water.
To date, there is no in situ treatment process known to the practice that result in a satisfactory in situ destruction of the alkyethers and TBA. There is need, therefore, for a practicable in situ technology that results in the satisfactory destruction of the target alkyl ether(s) in situ, and that does not require the withdrawal and above-ground treatment and discharge of fluids.
U.S. Pat. Nos. 5,750,364 and 5,811,010, assigned to Shell Oil Company, relate to a bacterial culture that aerobically degrades alkylethers and TBA to non-toxic carbon dioxide and water. However, in-situ remediation of sub-surface chemical contaminants by delivering contaminant-degrading bacteria to the sub-surface (this overall process will hereinafter be referred to as “bio-augmentation”) has not historically been embraced by the practice. This is mainly because most soils and aquifers already have sufficient numbers of microorganisms capable of causing substantial biodegradation of readily biodegradable compounds, and therefore addition of other organisms is not warranted. It is also accepted that delivering and maintaining non-indigenous microorganism cultures in the subsurface is a very difficult task.
Now, with the need to treat aquifers and soils contaminated with more recalcitrant chemicals, there is a need for a bio-augmentation process which can successfully deliver and maintain non-indigenous microorganism cultures in the subsurface. More specifically, there is a need for an effective bio-augmentation process for remediating oxygenate chemicals such as alkyl ethers, particularly MTBE, and TBA contamination in soils and groundwater.